Salt bath for carburizing and method of carburizing



Patented Apr. 27,1937

PATENT OFFICE SALT BATH FOR OARBURIZING AND METHOD OF CARBURIZING Henry H. High, Chicago, Ill.. assignor to E. F. Houghton and Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application March 19, 1936,

Serial No. 69,717

8 Claims. (Cl. 148-15) This invention relates to a novel salt bath for the case-hardening or carburizlng of iron and steel, and to a novel method of carburizlng,

and specifically it relates to a carburlzing salt bath of the so-called cyanide type containing an alkali metal cyanide and salts molten at the temperature of carburization, which bath due to its composition possesses many advantages over the baths of the general type previously used. 7

One object of the present invention'is to provide a salt bath with an intensified case-harden-' ing or carburizing effect, by which iron and steel articles may be treated to obtain on the surface thereof a carbon case of the desired depth and qualities. 3

A further object of my'invention is to provide a salt bath in which the loss ofcyanide by wasteful decomposition and evaporation is reduced to a minimum and which retains its effective case-hardening action for an" extended period of time merely by replacement of the small amount of salts removed from the bath by drag-out and evaporation.

Still another object of the invention is to provide a salt bath in which, due to its composition, fresh carbon available for the carburization reaction is continuously formed with the result that a bath is available of unequalled casehardening action and of extended active life.

Another object is to furnish a salt bath which, when molten, is provided with a surface layer of carbon which may be formed from a reaction of the ingredients of the bath and may be continuously maintained thereby so that evaporation of the bath components and oxygen absorption with its decomposing action are minimized, the carbon thus furnished being supplemented by carbon of particular characteristics, added to the bath.

Other objects, including the provision of a bath of high fluidity and of a novel method of case-hardening, will be apparent from a consideration of the "specification and claims.

Case-hardening or carburizing baths have long been known and employed for imparting a surface hardness to articles of iron or steel. These baths rely on the presence of a cyanogeri compound, usually a cyanide, for their action. In most instances, sodium cyanide is an ingredient of the initial bath due to its availability and cheapness, although a reaction in the bath may convert the sodium cyanide, or a portion thereof, into another cyanide as is encountered when barium chloride or the like is a component of the bath. The cyanogen compound is usually employed with a salt, or a mixture of salts, which is molten at carburizing temperatures and inert in the bath in the sense that it does not furnish cyanogen thereto. The salt or mix- -ture of salts selected is dependent on the temperature to be used in the case-hardening process and on the fluidity desired in the bath. The salt most usually employed is an alkali metal chloride, and a mixture of sodium chloride and potassium chloride is often recommended. Alkaline earth metal salts, for example barium or strontium chloride, may also be used, preferably in admixture with one or more alkali metal salts.

The baths described have well recognized limitations, for example the alkali metal and alkaline metal chlorides or other salts have the tendency to change during prolonged use into carbonates, for example sodium or barium carbonate, which gradually prevent the release of carbon from the components of the bath. Thus a bath may contain from 20% to 40% cyanide and yet produce little or no cementation. Since the formation of carbonates in the baths has 4 been known to retard or prevent the case-hardening process, carbonates, such as sodium and barium carbonates, have as a rule been considered not only of no advantage in the bath but of positively deleterious properties, the presence of which should be avoided.

Contrary to previous conceptions, I have found that if substantial quantities of barium carbonate, for example more than 15% of this compound, are included with the materials making up the salt bath, a bathis obtained with many novel and advantageous-properties. Barium oxide, although a more expensive product, may be employed, if desired, in place of the barium carbonate. Hereinafter, barium oxide is to be considered equivalent to barium carbonateand it is to be understood that barium oxide is included within the term, barium carbonate. The barium compound in the salt baths of the present invention serves as a very active agent in producing a carbon reaction, thus greatly intensifying the case-hardening effect and at the same time retarding the disproportionate release of cyanide. The bath of the invention produces continuously fresh carbon within itself, has an unequalled case-hardening action, and retains its cementation eil'ect for weeks so long as the small loss of material by drag-out and evaporation is replaced by fresh material in accordance with the usual practice. The

dailyloss of cyanide is less than half the loss usual with similar baths containing no barium carbonate. For example, the loss in fusing a bath of the present invention and containing 30% of sodium cyanide will be only about 3%, and the daily loss of the material will not exceed 1% to 2%. Furthermore, the case-hardening action of such a bath is superior to that of a bath containing 50% to 75% of sodium cyanide.

In a bath containing barium carbonate, a series of reactions are encountered which do not take place with the use of other salts. My investigations lead me to believe that the reactions may be represented by the following equations, although it will be understoodthat the invention is not to be limited thereby:

(a) BaCO3+heat=Ba0+CO2 (c) COz in presence of BaCz) (d) BaCz+2N=Ba(CN)z While with other barium salts, there is a tendency for barium carbonate to form permitting reaction (e) to take place, the use of barium carbonate is to be contrasted with the use of other barium salts, such as the halides. When the barium halides are employed, the cyanide content drops rapidly. With barium carbonate, however, since this compound is one of the end products of the carburizing process, it is a controlling factor in establishing an equilibrium in the bath and consequently the life of the salt bath is prolonged in the use of barium carbonate, thus enabling the heat-treater to carburize a greater number of pounds of steel per pound of salt bath used. Due to the fact that the bath retains its cyanide content when barium carbonate is used for a longer time than is otherwise the case, there is a higher percentage of free carbon in the bath thus producing a greater depth of case than can be obtained for a given time and temperature treatment when the cyanide content has been more or less dissipated. Furthermore, the physical violence of the bath such as bubbling, frothing, and fuming often encountered is obviated when barium carbonate is used due to the fact that this compound afiects the equilibrium of the molten bath. Its addition to the bath is similar to the addition of carbon dioxide to a mixture of explosive gases to retard the violence of the reaction between the gases upon combustion.

With the salt bath of the invention containing barium carbonate, a scum of carbon is formed, upon fusing of the bath, thickening into a heavy layer as the bath is maintained in molten condition. The layer of carbon thus formed provides a protective covering on the surface of the bath, impeding the entrance of oxygen from the atmosphere and thereby preventing the excesive loss of cyanide strength which is an undesirable feature of other cyanide cementation baths. To 11- lustrate the carbon-forming capacity of the bath of the invention, the carbon layer was entirely removed after its formation until the surface of the bath showed nothing but the red-glowing salt. The carbon formed again and when an iron covering was placed on the bath a layer similar to that removed was formed. Tests of the bath showed that it was equal in strength to the bath employed before the carbon removal. While the carbon layer is forming whether initially or after removal of the first-formed layer as described, the container of the bath is preferably provided with a cover, although this does not need to be air tight. The bath, however, does not require the use of any cover when the carbon layer has been formed.

The bath of the invention is also characterized by its high fluidity, and, therefore, the loss by the removal of the salt with the work, known as drag-out is reduced to a minimum. Also the surface of the work is bright and the salt inc'rustation is easily removed by washing.

I am aware that the use of a small amount of alkaline barium compounds, such as barium oxide, barium carbonate, etc., in baths containing large quantities of barium chloride has been suggested to retard the action of the molten bath. In that case, the percentage of barium compound employed was low since the bath in addition to the large quantity of barium chloride (at least four times the percentage of cyanide present) necessarily also contained adequate percentages of sodium cyanide and of salts'lowering the melting point of the bath. The use of barium compounds in this instance where retardation of the bath was desired is to be contrasted with the present invention where at least 15% of barium carbonate is employed to cause the formation of carbon in large quantities, to increase the carburizing power of the bath and to preserve its effectiveness for a practically unlimited time so long as the drag-out is replenished and the content of cyanide is not permitted to fall below The salt bath of the present invention characterized by the presence therein of at least of barium carbonate and at least 4% of activated carbon of low combustibility. The upper limit of barium carbonate which may be used in the bath is relatively immaterial, but for most purposes 50% to 60% of barium carbonate will not be exceeded. Generally, the barium carbonate content will be in the neighborhood of to The bath also contains the necessary cyanogen-furnishing compound, preferably sodium cyanide in amounts between 10% and 50%, generally between 15% and 45%. The balance of the bath is composed of a fusible salt or a mixture thereof which imparts to the bath the necessary properties such as the proper melting point and fluidity. It is to be understood that the amounts of these salts employed and the nature thereof are determined so that the desired properties are obtained, taking into consideration the amounts of barium carbonate, activated carbon of low combustibility, and cyanogen compound to be employed in the bath. Sodium or potassium chloride in a mixture thereof may be mentioned as typical of these salts.

The use of barium chloride in the bath gives an especially desirable reaction since barium cyanide is formed by transformation of the sodium cyanide into sodium chloride. The transformation of barium chloride and sodium cyanide into barium cyanide is slowed up but the formation of carbon is increased when the bath contains barium carbonate as herein' described. A very satisfactory bath, therefore, due to the influence of the barium carbonate and barium chloride on each other, contains both compounds, the barium chloride being present generally in amounts from 15% to 30%, although obviously the amount of this compound may be reduced to a few percentage in which case some beneficial results are obtained. The percentage may also exceed 30% but this is not recommended, since satisfactory results are obtained without the use of an excess amount of barium chloride.

In the salt bath of the present invention, as

previously stated, there is also included at least 4% of activated carbon of low combustibility. The upper limit is not critical but percentages higher than 10% to are not required. This activated carbon of low combustibility cooperates with the barium carbonate, so that particularly desirable results are obtained. The activated carbon of low combustibility partially oxidizes and this furnishes the carbon monoxide necessary for reaction (e). It also supplements and strengthens the carbon crust formed on the top of the bath which holds the carbon monoxide in contact with the liquid salts and prevents an excessive volatilization of the salts. Furthermore, it provides an excess of free carbon in the bath for use in reaction (2)) and in the formation of the carbon monoxide required in the carburizing action, thus preventing the carbon liberated in reaction (e) from being thus consumed. The acti-.

vated carbon of low combustibility functions in a bath containing substantial amounts of barium carbonate in a markedly difierent manner than it functions in a bath containing merely inert fusible salts such as sodium or potassium chloride, and it is to be noted that the carbon monoxide furnished by the activated carbon of low combustibility reacts with the barium cyanide formed and not with the sodium or potassium salts in the bath, thus a definite cooperation is encountered between the barium carbonate and the activated carbon of low combustibility.

The activated carbon employed is characterized by itslow combustibility, which property has been imparted thereto by special physical or chemical treatment. The manufacture of such carbons is described in German Patents No. 435,972 of December 28, 1927, No. 466,358 of October 5, 1928, and No. 488,669 of January 3, 1930. While the processes of these patents result in carbons which are activated and are of low combustibility, a carbon of such characteristics may be obtained by other processes. Regardless of how obtained, the carbons applicable for use are termed herein and in the claims activated carbon of low combustibility. These carbons, upon being subjected to the full flame of a Bunsen burner for a period of two hours, leave a residue of non-combustibles of at least 50% by weight of the original sample as distinguished from the ordinary activated carbon, which, upon subjection to the same test, leaves a considerably smaller percentage of non-combustibles.

In accordance with the disclosure of German Patent No. 453,972, ordinary carbon from any source is heated to glowing in a suitable vessel with porous or gas-permeable walls, and thereafter submitted while glowing to the action of oxygen-containing gases passed through the carbon mass or through the walls of said vessel. The German Patent No. 466,358 describes an improvement of the process of German Patent No. 453,972 by which even less combustible activated carbon is prepared by treating granular carbon fro-m any source with a water-glass solution fol-'- lowed by precipitation of the silicic acid by the action of acids prior to the treatment of the carbon as described in the main German Patent No. 453,972. An activated carbon product having a silicic acid skeleton is produced which possesses great mechanical strength and may be used advantageously in chemical reactions without losing its shape. The German Patent No. 468,669 describes another method for manufacturing a highly activated carbon of low combustibility. The carbon is treated with inorganic substances in solution, or as colloids, which substances exert a combustion-decreasing influence, and are precipitated as insoluble oxides by a carbonlzation process. Prior to carbonization, other substances are-also added, such as zinc chloride or phosphoric acid to increase the activity of the carbon.

The bath may be prepared by simply mixing the ingredients and thereafter heating the mixture to fusion. During the initial fusion, the container is advantageously covered until the carbon film has formed. The salts are preferably first mixed and fused and an agglomerate of activated carbon and the salts bound together by dextrine is then added. The-agglomerate is preferably formed by mixing a portion of the salts of the bath (other than cyanide) in a mixer with a little water, followed by the addition of the activated carbon. A solution of dextrine in water is then added at a rate as fast as it is taken up by the solid material. The mixing is then continued until the particles have agglomerated to about the size of a pea, after which it is drawn out and dried.

The following examples illustrate the invention:

Example I Per cent Sodium cyanide--- 15 Barium carbonate Barium chloride 29 Sodium chloride 26 Activated carbon of low combustibility 5 At 900 C., a depth of case of .6 mm. is obtained in one hour and of 1 mm. in two hours when 2.

- steel of .15% carbon is treated in the above bath The daily loss of cyanide is less than-2%.

Example II A salt bath containing barium carbonate, sodium cyanide, sodium chloride, and potassium chloride is heated to the carburizing temperature, for example 1650 F. While still molten, the pellets herein described containing the activated carbon of low combustibility are added, the bath af ter the addition of the pellets containing approximately 22% barium carbonate, 25% sodium cyanide, 38% sodium chloride, 10% potassium chloride, and 5% activated carbon of low combustibility. The bath is quiet from the initial melt. There is a minimum amount of fuming and the reaction does not become violent even after extended use, for example for a period of six weeks or more. Analyses of samples taken over such an operating period have shown that the sodium cyanide content did not vary over 1.5% and that the barium carbonate remained constant within 3%. The carbon content of the bath, due to the carbon added and that formed by the reactions, was maintained during the use of the bath. These results demonstrate the effectiveness of the barium carbonate and activated carbon of low combustibility in controlling the bath and of furnishing a salt bath of remarkable potency for the carburizing of iron and steel.

Example III A salt bath containing barium carbonate, sodium cyanide, barium chloride, sodium chloride, and potassium chloride is heated to the carburizing temperature, for example 1,650 E, and while still molten the pellets containing the activated carbon of low combustibility are added thereto. The approximate composition of the bath after the addition of the pellets is as follows: barium I tibility carbonate 15%, sodium cyanide 35%, barium chloride 15%, sodium chloride 22%, potassium chloride 8% and activated carbon of low combus- This bath reacted more vigorously than that of the preceding example, and while the life of the bath is not as long as that of the other example, a higher percentage of free carbon is available due to the presence of barium chloride which resulted in a greater penetration of the case.

Considerable modification is possible in the percentage of materials employed, and in the nature of the ingredients of the bath, as well as in the temperatures employed in case-hardening without departing from the essential features of the invention.

I claim:

1. A salt bath for case-hardening iron and steel which comprises sodium cyanide, barium carbonate in quantities greater than about 15%, activated carbon of low combustibility greater than 4%, and a salt molten at the case-hardening temperature.

2. Asalt bath for case-hardening iron and steel which comprises sodium cyanide from 10% to 50%, barium carbonate from about to 50%, activated carbon of low, combustibility from 4% to 15%, and sufiicient amount of a salt fusible at the case-hardening temperature to render the bath molten.

3. A salt bath for case-hardening iron and steel which comprises sodium cyanide from 15% to 45%, barium carbonate in the neighborhood of 25% to 30%, activated carbon of low combustibility from 4% to 15%, and suflicient amount of a salt fusible at the case-hardening temperature to render the bath molten.

4. A salt bath for case-hardening iron and steel which comprises sodium cyanide from 10% to 50%, barium carbonate from about 15% to 50%, barium chloride, activated carbon of low combustibility from 4% to 15%, and a sumcient amount of a salt fusible at the case-hardening temperature to render the bath molten.

5. A salt bath for case-hardening iron and steel which comprises sodium cyanide from 15% to 45%, barium carbonate from about 15% to 50%, barium chloride from about 15% to 30%, activated carbon from about 4% to 15%, and a suflicient amount of a salt fusible at the casehardening temperature to render the bath molten.

6. The method of case-hardening which comprises subjecting iron and steel articles toimmersion at case-hardening temperatures in a molten bath comprising sodium cyanide, barium carbonate in quantities greater than about 15%, activated carbon of low combustibility greater than 4%, and a salt molten at the case-hardening temperature until the desired case is obtained, and thereafter removing said articles from said bath.

7. A salt bath for case-hardening iron and steel which comprises sodium cyanide in the neighborhood of 15%, barium carbonate in the neighborhood of 25%, barium chloride in the neighborhood of 29%, activated carbon of low combustibility in the neighborhood of 5%, and suflicient amount of a salt fusible at case-hardening temperature to render the bath molten.

8. A salt bath for case-hardening iron and steel which comprises sodium cyanide in the neighborhood of 25%, barium carbonate in the neighborhood of 22%, activated carbon of low combustibility in the neighborhood of 5%, and suflicient amount of a salt fusible at case-hardening temperature to render the bath molten.

HENRY H. HIGH. 

